Now I've separated the 84 page document into 3 separate ones and that works, so the main cause must be memory of my computer. Good to finally understand for future projects. I was about to give up on Scribus because I couldn't find the issue.
Thanks RodneyLee for the kind replies and patience with a rookie.
For future Scribus beginners take this potential root cause into consideration so you don't have to search endlessly what the problem is.